1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Wireless communication systems typically use radiofrequency signals to convey information over an air interface between transmitters and receivers. For example, a base station (or eNodeB) may communicate with user equipment (UE) using transceivers implemented in the base station and the user equipment. The simplest transceivers use a single antenna to transmit and receive the radiofrequency signals. However, more advanced transceivers can use more than one antenna for transmission and reception of signals transmitted over the air interface. For example, base stations can employ arrays of 2, 4, 8, or more antennas for transmitting and receiving radiofrequency signals over the air interface. User equipment can also implement more than one antenna. Systems that employ multiple antennas on the receiver side and/or the transmitter side are generally referred to as multiple-in-multiple-out (MIMO) communication systems. MIMO systems may also be implemented as single-user MIMO (SU-MIMO) systems or multiple-user MIMO (MU-MIMO) systems.
The wireless communication channels in a MIMO system are defined by a channel matrix that determines the signal strength received at the receiver-side antennas as a function of the signal strength transmitted by the transmit-side antennas. The channel matrix is therefore a function of the transmitter and receiver antenna configurations, as well as the scattering environment between the transmitter and the receiver. The dimensions of the channel matrix are determined by the number of transmitter-side antennas and receiver-side antennas. Cross-antenna interference, which is represented by the non-diagonal elements of the channel matrix, can in theory be removed by pre-coding transmitted signals to diagonalize the channel matrix. For example, a pre-coding matrix that diagonalizes the downlink channel matrix could be determined for each UE using a conventional eigenvalue/eigenvector decomposition of the channel matrix for symmetrical channel matrix or singular value decomposition of the channel matrix for asymmetrical channel matrix. However, defining a precise pre-coding matrix for each UE requires sufficient feedback from the UE to exactly determine the downlink channel matrix, as well a sufficient computing power to compute the pre-coders in real time. In practice, constraints on the uplink channel overhead and transceiver design make this impossible.
Conventional MIMO systems therefore use a codebook that includes a predetermined quantized set of pre-coding matrices. The codebook includes a set of pre-coding matrices that diagonalize an ideal channel matrix defined for a specific antenna configuration and a non-scattering environment. The transmitter can then choose one of the pre-coding matrices based on feedback received from the receiver. For example, a UE can feedback channel state information that can be used to select a pre-coding matrix to apply to signals transmitted over the downlink to the UE. Exemplary channel state information (CSI) includes channel quality information (CQI), a pre-coding matrix indicator (PMI), a rank indicator, a pre-coding type indication (PTI), and the like. The CQI typically represents the recommended modulation scheme and coding rate that should be used for the downlink transmission, the RI provides information about the rank of the channel and can be used to determine the optimal number of layers that should be used for downlink transmission in spatial multiplexed systems, and the PMI indicates which pre-coding matrix to use, e.g., in closed loop spatial multiplexing systems. The dimensions of the pre-coding codebook are constrained by the control signaling overhead available for providing the necessary feedback.
Codebooks are typically standardized for one assumed antenna configuration, such as a cross-polarization antenna array or a linear antenna array. For example, on the downlink, codebooks for linear arrays of 2, 4, and 8 TX antennas have been standardized for the Rel-10 LTE technical specification to support SU-MIMO and MU-MIMO. The eNodeBs and UE that operate in accordance with the standard must use these codebooks for transmission over the air interface, regardless of their actual antenna configurations. Furthermore, the CSI feedback mechanism design for DL MIMO is based on the standardized codebooks and each UE assumes that the signals it has received were generated by an eNodeB that implements the assumed antenna configuration. For another example, on the uplink, the pre-coding vector assigned to the UE for UL SU or MU-MIMO is based on codebooks for 2 and 4 TX antennas that are defined in the specification established by the Third Generation Partnership Project (3GPP). The eNodeB will assign a codebook from the specified set to each UE regardless of the actual antenna configuration used by the UE.